1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane mode liquid crystal display device.
2. Discussion of the Related Art
A liquid crystal display (LCD) device controls an electric field applied to a liquid crystal cell to modulate light incident to the liquid crystal cell, thereby displaying a picture. The LCD device can be classified into a vertical electric field type and a horizontal electric field type in accordance with a direction of an electric field generated to drive liquid crystals in the liquid crystal cell.
The vertical electric field type LCD device includes a pixel electrode and a common electrode vertically opposed to each other on a first substrate and a second substrate, respectively, which are also vertically opposite to each other. When a voltage is applied to the electrodes, an electric field in a vertical direction is generated and applied to the liquid crystal cell. The vertical electric field type LCD device generally provides a relatively wide aperture ratio. However, the vertical electric field type LCD device generally has a narrow viewing angle. A typical liquid crystal mode of the vertical electric field type LCD device is a twisted nematic mode (hereinafter, referred to as “TN mode”).
In the TN mode, liquid crystal molecules 13 are located between a first glass substrate 14 and a second glass substrate 12, as shown in FIGS. 1A and 1B. A first polarizer 15 having a light transmission axis of a specific direction is formed on a light exiting plane of the first glass substrate 14. Similarly, a second polarizer 11 of the light transmission axis, which perpendicularly crosses the light transmission axis of the first polarizer 15, is formed on the light incident plane of the second glass substrate 12. Further, in the TN mode, a transparent electrode (not shown) is formed on each of the first and second glass substrates, and an alignment film (not shown) is formed to set a pre-tilt angle.
The operation of the TN mode is described as follows using a normally white mode TN mode LCD device as an example. When there is no voltage applied to the transparent electrodes (i.e., inactive state), local optical axes (i.e., director) of the liquid crystal molecules in a liquid crystal layer are continuously twisted by 90° between the first glass substrate 14 and the second glass substrate 12. Therefore, polarized direction of a linearly polarized light incident through the polarizer 11 of the second glass substrate 12 follows the optical axes of the twisted liquid crystal molecules, thereby passing through the polarizer 15 of the first glass substrate 14 as shown in FIG. 1A. Hence, the LCD device is normally in a “white” state when no voltage is applied.
In contrast, when a voltage is applied to the transparent electrodes (i.e., active state), an electric field is generated by the voltage difference between the transparent electrodes. The generated electric field forces the normally twisted liquid crystal molecules 13 to align in the direction of the electric field, thereby becoming untwisted. As a result, the light axes of a central part of the liquid crystal layer become parallel to the electric field. As the linearly polarized light incident through the polarizer 11 passes through the untwisted liquid crystal layer, its polarized direction remains intact. Hence, the linearly polarized light is blocked by the first glass substrate 14 as shown in FIG. 1B.
In the TN mode, a wide viewing angle is difficult to achieve because its contrast ratio and brightness vary significantly in accordance with the viewing angle. In general, horizontal electric field type LCD device has a wider viewing angle than the vertical type TN mode LCD device. A representative liquid crystal mode of the horizontal electric field type LCD device is an in-plane switching mode (hereinafter, referred to as “IPS mode”).
In the IPS mode, an electric field is generated between electrodes formed on the same substrate and the liquid crystal molecules are driven by the in-plane electric field. In the IPS mode as shown in FIG. 2, a pixel electrode 21 and a common electrode 22 are formed on the same glass substrate. Accordingly, a wide viewing angle is achieved because a liquid crystal 23 is substantially driven within a horizontal plane by the electric field applied between the electrodes 21 and 22.
FIG. 3 shows a schematic diagram illustrating an array arrangement of an IPS mode according to the related art. As shown in FIG. 3, an IPS mode LCD device includes a thin film transistor (TFT) substrate 30 on which pixel electrodes 21 and common electrodes 22 are formed, and a drive circuit 28 to supply a common voltage Vcom to common wire lines 24 and 25 of the TFT substrate 30. A plurality of data lines 27 and a plurality of gate lines 26 cross each other on the TFT substrate 30, and a TFT 23 is formed at each crossing part thereof. First common wire lines 24 are formed in a horizontal direction and connected to the common electrodes 22. Second common wire lines 25 are formed in a vertical direction and interconnect the first common wire lines 24 to the drive circuit 28. A source electrode of the TFT 23 is connected to the data line 27, a drain electrode is connected to a pixel electrode 21, and a gate electrode is connected to the gate line 26.
The drive circuit 28 converts digital image data into analog data voltages to be supplied to the data lines 27. The driving circuit 28 also supplies a common voltage Vcom to the second common wire lines 24. The common voltage Vcom supplied through the second common wire lines 24 is supplied to the common electrodes 22 through the first common wire lines 24. The liquid crystal cells are driven by an effective potential caused by a difference between the common voltage Vcom applied to the common electrodes 22 and pixel voltages applied to the pixel electrodes 21, thereby modulating light.
To reduce the data voltage in the IPS mode, a line inversion system is used where the data voltage of the same polarity is supplied to the liquid crystal cells of the same horizontal line while the data voltage of opposite polarities is supplied to the vertically adjacent liquid crystal cells. The common voltage Vcom of the line inversion system is generated as an AC voltage, which is inverted to a high voltage and a low voltage for each one horizontal period to reduce the swing width of the analog data voltage supplied to the data line 27.
In such an IPS mode LCD device, if a gap between the pixel electrode 21 and the common electrode 22 is lengthened to increase the aperture ratio, the effective potential of the liquid crystal cell needs to be increased accordingly by either increasing the data voltage or the common voltage Vcom. However, doing so increases the cost of the drive circuit while also increasing the power consumption of the device.